How do I check if an element is undefined?

Question

I would like to check to see if a particular attribute of a DOM element is undefined - how do I do that?

I tried something like this:

if (marcamillion == undefined) {
    console.log("Marcamillion is an undefined variable.");
}
ReferenceError: marcamillion is not defined

As you can see, the reference error is telling me that the variable is not defined, but my if check is clearly not working, because it is producing the standard js ReferenceError as opposed to the error message I am looking for in my console.log.

Edit 1

Or better yet, if I am trying to determine if the attribute of an element is undefined like this:

$(this).attr('value')

What would be the best way to determine if that is undefined?

possible duplicate of [How to determine if variable is 'undefined' or 'null'](http://stackoverflow.com/questions/2647867/how-to-determine-if-variable-is-undefined-or-null) and http://stackoverflow.com/questions/27509/detecting-an-undefined-object-property-in-javascript — dsgriffin, Mar 26 '13 at 09:35

romainberger · Accepted Answer · 2013-03-26T09:46:01.380

26

Using typeof:

if (typeof marcamillion == 'undefined') {
    console.log("Marcamillion is an undefined variable.");
}

Edit for the second question:

if ($(this).attr('value')) {
    // code
}
else {
    console.log('nope.')
}

edited Mar 26 '13 at 09:46

answered Mar 26 '13 at 09:35

romainberger

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Say I have an attribute that I am trying to test the value of - i.e. `$(this).attr('value')`...what would be the best way to determine if that is undefined? – marcamillion Mar 26 '13 at 09:42
1

Simply doing `if ($(this).attr('value'))` will work. – romainberger Mar 26 '13 at 09:44
How do I detect the `not` version of that? In ruby, I would just add a `!` infront of the condition, but I am getting an error now....or is that not possible here? – marcamillion Mar 26 '13 at 09:49
1

`if (!$(this).attr('value'))` should work. What error do you get? – romainberger Mar 26 '13 at 09:52
Strange...I got an error before. Not quite sure why...but thanks! – marcamillion Mar 26 '13 at 09:53

score 7 · Answer 2 · answered Mar 26 '13 at 09:37

7

if (typeof marcamillion === 'undefined') {
    console.log("Marcamillion is an undefined variable.");
}

Note that using === instead of == is considered better style.

answered Mar 26 '13 at 09:37

Fabien

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Say I have an attribute that I am trying to test the value of - i.e. `$(this).attr('value')`...what would be the best way to determine if that is undefined? – marcamillion Mar 26 '13 at 09:43

How do I check if an element is undefined?

2 Answers2